Process for electrodeposition of a dispersion of finely divided substances in an apolar dispersing agent

ABSTRACT

A dispersion of finely divided substances in an apolar dispersing agent, to an electrophoretic method using such a dispersion and to an electrophotographic method using such a dispersion as a developer.

United States Patent [191 Hansen et a1.

[11] 3,920,532 [451 Nov. 18, 1975 [54] PROCESS FOR ELECTRODEPOSITION OFA DISPERSION OF FINELY DIVIDED SUBSTANCES IN AN APOLAR DISPERSING AGENT[75] Inventors: Norbert Ernst Fritz Hansen;

Siegfried Stotz, both of Aachen,-

Germany [73] Assignee: U.S. Philips Corporation, New

York, NY.

[22} Filed: July 6, 1973 21 Appl. No.; 377,068

Related US. Application Data [62] Division of Ser. No. 43,223, June 3,1970, Pat. No.

[30] Foreign Application Priority Data June 6, 1969 Germany 1928817Primary Examiner-Howard S. Williams Attorney, Agent, or Firm-Frank R.Trifari; Norman N. Spain [57] ABSTRACT A dispersion of finely dividedsubstances in an apolar dispersing agent, to an electrophoretic methodusing such a dispersion and to an electrophotographic method using sucha dispersion as a developer.

2 Claims, No Drawings PROCESS FOR ELECTRODEPOSITION OF A DISPERSION OFFINELY DIVIDED SUBSTANCES IN AN APOLAR DISPERSING AGENT This is adivision of application Ser. No. 43,223, filed June 3, 1970, and nowU.S. Pat. No. 3,766,125.

Electrophoresis is understood to mean the transport of dispersedparticles in an electric field, which transport is brought about becausethese particles carry an electric charge. Many uses of electrophoresisin polar dispersing'agents are known in technology. Charging of theparticles is based on the formation of 'ions by electrolyticdissociation in polar dispersing agents having a high dielectricconstant such as water or alcohols whose dipolar molecules surround theion. The solvatation energy of this process is equal to or larger thanthe lattice energy of an ionic compound and 'renders the dissolution ofsuch a compound in the dispersing agent possible. To obtain a charge thecolloidal particle itself must include dissociable groups, for example,macromolecules having built-in carboxyl or hydroxyl groups, or suchparticle absorbing an ion of a low molecular weight electrolyte. Themechanism of electrophoresis in polar dispersing agents is based onmethods of depositing finely divided substances on objects which forthis purpose are arranged as electrodes and have a conducting surface.Known technical uses are among others coating oxide cathodes andlacquering motor bodies and other objects.

In certain cases there are great limitations in using the method for theelectrophoretic deposition of particles. Substances which are soluble inwater or react chemically in a different manner cannot be used inaqueous systems. The comparatively high natural con ductivity of thesesystems bars their use in those cases wherein only a limited quantity ofthe charges generating the field is available and wherein as great apossible mass for each transported charge must be deposited.

Furthermore it has already been proposed to use apolar organic liquidshaving a small dielectric constant and a high electric resistance asdispersing agents. The formation of ions by means of dipolar solvatationin the manner described above is not possible in these dispersingagents. Small solid-substance particles may, however, be charged bybarrier layer effects, triboelectrical effects or contact potentialdifferences, a particle having a dielectric constant which is higherthan that of the dispersing agent being positively charged and beingnegatively charged in the converse case. This natural charge may besufficient for small particles (approximately 0.1 pm) for transport inan electric field;

on the other hand larger particles cannot be deposited on a flatelectrode. In this case the so-called dielectrophoresis innon-homogeneous electric fields occurs. This occurs in such a mannerthat an uncharged or a weakly charged particle in the electric field ispolarized. In the non-homogeneous field a net attraction of gravitytowards a higher field strength is obtained independent of the polarityof the electrode, when the dielectric constant or the polarisability islarger than that of the surrounding dispersing agent. The attractiveforce is proportional to the polarisability and to the volume of theparticle and the gradient of the square of the field strength. Largerand insufficiently charged particles can thus be deposited on the edgesof the electrodes and on irregularities and on corners and edges ofcharging profiles because non-homogeneous field distributions occur inthese areas.

. 2 For a uniform electrophoretic deposition of particles larger than0.1 pin, up to approximately 10 gm as occur in most cases in ahomogeneous electric field between two electrodes in apolar media itmust be attempted to find a possibility to sufficiently charge theseparticles. The same applies to the deposition of pigments which areproportional to the charge on differently charged dielectric surfaces asoccur in the latent charge image of an image-reproducing layer inelectrostatic electrophotography. In that case it must be possible tocheck and control the charge of the particles satisfactorily in order todeposit by means of an optimum ratio between charge and mass as many aspossible particles per surface charge and to obtain a smoothelectrophoretical behaviour.

German Pat. No. 1,047,616 describes a method of developing electrostaticcharge images wherein a dispersion of pigment particles in apolar mediasuch as cyclohexane is used. The pigment particles are electricallyrendered selective for the image charge by adding a substance whichcontrols the pigment. This substance gets to surround the particle andto influence charging by adjusting a given ratio between its dielectricconstant and the dielectric constant of the solvent. Such a substanceis, for example, an alkyd resin, an alkyd resin modified with linseedoil or boiled linseed oil itself. The two first-mentioned compounds mustcharge the pig ment particle positively while the last-mentioned comcessthey are hardened. In that case a known hardening agent, namely a soapof a heavy metal such as lead or cobalt naphthenate is used. The agentis already added to the developer solution.

In addition more has become known about the stability of suspensions inapolar media. On the one hand the steric or entropic stabilizationeffect is known and on the other hand the stabilization by electrostaticrepelling of particles charged at the same polarity. Thus, suspensionsmay be stabilized both with ionic and nonionic substances from the groupof surface-active substances. It was found that solutions of ionicstabilizers 1 have an electric conductivity which is 10 times higherthan that of the pure apolar solvents. Thus it must be possible to formions also in apolar media, even at a very slight degree of dissociation.For an ionic sub stance in an apolar solvent, it must be assumed that asa result of the structure of the substances having these effectsmicelles are formed from a lyophilic organic acid radical and alyophobic cation, which micelles may envelope and shield a few lyophobiccations, while a few lyophilic radicals are present in the solution. Ifadditional particles are present in a suspension the lyohpobic cationcan be adsorbed in polar groups at the surface of the particles. Thecharge may thus be regarded to be a dissociative adsorption wherein acertain type of ion is bound in an adsorption phase while the other ionis present in the solvent.

The experiments discussed in the preceding paragraphs were performed incomparatively strong solvents such as benzene or xylene on fairly smallparticles l um) having polar surfaces (oxides). However, it was foundthat no satisfactory electrophoretic deposition'in the sense of theinvention could, be obtained under the said circumstances. The followingdifficulties occur among others:' due to still unexplained conditions onthe electrodes it is difficult to obtain an adherent deposition to theelectrodes with the aid of the particles transported by electrophoreticprocess.

For electrophotographic and electrographic techniques all particles mustat an average carry the same charge of a certain polarity becauseotherwise the pigmentation proportional to the charge is not assured anddeposition occurs in unwanted areas. In addition to charged particles,insufficiently charged particles frequently occur in comparatively largeparticles so that not only electrophoresis but also dielectrophoresis isobserved. As a result strong potential differences as occur at the edgesin the image are particularly strongly pigmented.

Ions of the polarity which are adsorbed at the solid particle may alsooccur in micelles so that the charge image for the deposition cannot beutilized optimally because the micelles neutralize the charge unused.

An object of the present invention is to provide a dispersion of solidparticles in apolar solvents which does not give the above-mentioneddrawbacks and wherein solid particles of an arbitrary material naturesuch as metals, semiconductors and dielectrics having a grain size ofapproximately 0.01-10 um and in special cases even larger particles arecharged.

According to the invention the dispersion of a finely divided substancein an apolar dispersing agent having an electrical conductivity which issmaller than 10 9 cm' is characterized in that the dispersion includessurface-active ion-forming substances which are soluble in thedispersing agent and are dissociable in such a manner that theirconductivity in a 10' molar solution is greater than 10" *cm andpreferably between 1 X and l X 10 0 "cm and that these substancescomprise bivalent or multivalent ions.

Such a multivalent compound dissociates into an ion consisting of themetal atom and the remaining lyophilic acid radical and into anoppositely charged lyophilic radical itself. A metal ion including aremaining lyophilic acid radical formed in this manner may be stabilisedeither into a micelle or into an adsorption phase built up similarly inan easier manner than a single alkali ion. Alternatively mixtures ofseveral of these substances may yield particularly favourable results.The concentration of the substance to be added must lie between certainlimits: When this concentration is toolow dielectrophoresis occurs andwhen it is too high the charges of a charge pattern are compensated bymicelles of the same polarity as the pigment particles so that only asmall deposit thickness can be obtained. A

. series of simple tests enables anyone skilled in the art to determinethe active concentration range for each individual case.

4 acrylic acid esters having a molecular weight of 10 -10, polyacrylicacid esters, polyalkyl styrenes, polyvinyl alkyl ethers and copolymersor polymethacrylates having cyclic amides and fumarates.

By adding such a macromolecular compound which is denoted by the termintensifying and stabilising agent" it is possible to render potentiallyionic substances usable by which a usable suspension as such for anelectrophoretic deposition within the scope of the present inventioncannot be manufactured. Dispersions can be stabilised in an optimum andreproducible manner with these substances. In this manner surfacecoatings of more than 2 mg/cm can be obtained by means of anelectrophotographic technique. The usual quantities ofelectrophotographically deposited pigment are slightly more than 0.1mg/cm It is alternatively possible tocharge one and the same pigmenteither unambiguously positively or unambiguously negatively by usingdifferent ionic substances.

The quantities to be used are dependent on the concentration of theparticles, the nature and the size of their surface and on the manner ofdispersing. As a result of the synergistic action of the two additions aquantity of ionic substance is used which is much smaller than when onlythe last-mentioned substance is used. The intensifying and stabilisingagent is first dispersed with the pigment, optionally together with thepigment and the added ionic substance, The optimum activity frequentlycannot be obtained if this substance is added to the suspension at alater stage.

Ionic substances which are soluble in isoparaffins are, for example,:allkaline earth alkyl sulfonates containing 24 32 carbon atoms such asCa(SO C H basic alkaline earth alkyl sulfonates such as R-SO BaOCO+ BaSOR wherein R may be a mixture of different radicals, salts of fatty acidssuch as Mg, Ca and Ba oleates, Co-naphthenate, salts of alkyl salicylicacid such as calcium diisopropyl salicylate, alkaline earth and aluminumsalts of alkyl esters of sulphosuccinic acid, such as Ca-dodecylsulphosuccinate or cetyl-, octyl or stearyl titanate. Mixtures of thesesubstances may alternatively be active.

Particularly active combinations of these ionic substances are mixturesof an alkyl salicylate including cetyl titanate or an alkyl salicylateincluding an alkyl sulphosuccinate.

A dispersion has already been described hereinbefore wherein pigmentparticles are dispersed in cyclohexane while adding an alkyd resin,,analkyd resin modified with linseed oil or linseed oil itself including Pbor Co-naphthenate as a hardening agent, which, however, is not based onthe recognition of the present invention. These combinations aretherefore explicitlyexcluded from the rights applied for.

The dispersion according to the invention is described hereinafter withreference to a few Examples.

EXAMPLE 1 A mixture of solid substances having the followingcomposition:

0.5 g of Zn-activated ZnCdS having a grain size of approximately 1.5 pm.

0.015 g of a mixture of Ca-alkyl salicylate and Cadodecylsulphosuccinate 0.08 g of a copolymer of polymethacrylate includingcyclic amides having a molecular weight of several times 10',

was dispersed in 100-300 g'ms' of isoparaffin (C ,C, having a boilingrange of between 180 and 210C. After electrophoresis at a field strength'of 50-100 V.cm this dispersion yielded very homogeneous, compact layershaving a smooth surface These layers are suitable for luminescentscreens having a high resolving power. v i

In this method the solid substance may be replaced by any othersubstance while the nature and the concentration of the additions may bechanged and adapted to the envisaged object and to the condition of thesolid substance. Luminescent substances which are soluble in water suchas, for example, cesium iodide may be deposited advantageously in thismanner. Metals, for example, silver may be deposited for intensifyingmetallic conductive paths. Ceramic and metallic pulverulent magneticmaterials may be used for the manufacture of magnetic layers andstructures.

EXAMPLE 2 An electrophographic developer was prepared by dispersing thefollowing mixture:

1 g of gas soot which was superficially rendered oxygen-containing bypost-oxidation and had a particle size of approximately 0.1 urn in thedeveloper; primary particle size 0.03 pm,

0.8 g of basic Ba sulphonate of C l-I in 1000 gms of isoparaffin of thequality according to Example 1. Very fine-grained images were obtainedwith this very sensitive developer. Zinc oxide papers which are usuallycharged to several hundred Volts of surface potential by means of acorona discharge could be developed to full density even at 30-70 V.This technique is a condition for a high quality reproduction of halftones. Zinc oxide papers having a great charge produced stained andmottled images as a result of an non-homogeneous charge distribution andadditionally showed a steep gradation.

When being used as a negative developer the exposed areas are pigmentedon the negatively charged zinc oxide; thus this developer is suitablefor a negative-positive process.

Positively charged soot developersare very difficult to manufacture.Usually soot-pigmented synthetic resins or pastes having a high contentof synthetic resin are used as basic material for such developers. Sucha developer may, however, be manufactured easily from pure soot havingfew additions with the aid of the stabilising agents. This developerhas, for example, the following composition:

1 g flame soot 0.1 g Ca-diisopropyl salicylate I 0.4 g polymethacrylicacid alkyl ester having side chains of -20 C-atoms and a molecularweight of approximately 7% X 10" dispersed in 1000 gms of isoparaffin ofthe quality according to Example 1.

In many technological processs photographic methods are used to obtainpatterns on carrier materials by means of additive or subtractivemethods (photoetfrh-f ing, photohardening method). It is evident toobtain the pattern in the form ofa charge image for additive methodswhich image may be obtained electrophotographiabove-described method,strict requirements are imposed on the optimum controllable chargingcapacity of the particles.

An interesting use of the dispersion according to the invention consistsin the manufacture of the luminescent layer on the inner side oftelevision picture tubes and particularly of a pattern comprising threeluminescent materials for the screens of colour television picturetubes. For this purpose the inner side of the picture screen is providedwith a conducting layer and a photoconducting organic layer is providedon said layer; an electric charge is supplied by a corona discharge tothe photoconducting layer whereafter the charged layer is exposed inaccordance with the desired pattern and is treated with the liquiddispersion of luminescent materials according to the invention. Thisprocess is repeated for the two other luminescent materials. Theconducting layer and the photoconducting layer are finally removed byheating while simultaneously the luminescent layers are compacted bysintering.

The following dispersions are described as Examples for the manufactureof screens for colour television picture tubes according to theabove-mentioned methods.

A mixture having the following composition:

3 g activated ZnS (blue luminescent material) grain size approximately 5pm,

0.03 g of a mixture of Ca-diisopropyl salicylate and cetyl titanate(l 1) 0.5 g of a polyacrylic acid alkyl ester, molecular weight severaltimes 10 was first dispersed in:

g isoparaffin (C -C boiling range 4060C),

while using ultrasonic treatment, whereafter the dispersion obtained wasdispersed in 250 gms of isoparaffin (C -C boiling range 2 l 0C) and wasdiluted to 1,500 mls. for use. The dispersing was carried out by anultrasonic treatment so as to expose the luminescent material as littleas possible to mechanical load which reduces its light output. Toinfluence the cavitation for obtaining an effective irradiation in afavourable sense, a low boiling isoparaffin was used first.

A mixture of the following composition:

3 g activated YVO (red luminescent material) grain size approximately 5um,

0.003 g of a mixture of Ca-alkylsalicylate and Ca-dodecylsulphonate.

0.2 g of a polymethacrylic acid alkyl ester having side chains of C Hand a molecular weight of 8 X 10 was dispersed in isoparaffin likewiseas the blue luminescent material. In this dispersion the particles werepositively charged.

The following mixture was used for the dispersion of a negativelycharged (green) luminescent material:

3 g activated ZnCdS, grain size approximately 5 pm,

0.2 g Ba-oleate 0.3 g of a polyalkyl styrene having alkyl side, chainsof more than 4 carbon atoms.

What is claimed is:

7 1. ln an'electrophoretic method of manufacturing luminescent picturescreens the improvement which comprises employing a dispersion in amixture of isoparaffins apolar dispersing agents having an electricalconductivity of less than 10' cm. of finely divided particles capable ofbeing deposited on an electrode by electrophoresis and having a particlesize of about 0.01- am, at least one surface-active ionformingsubstance, said surface-active ion-forming substance being soluble insaid dispersing agent, having a dissociation capacity such that itselectrical conductivity in a l0 molecular solution lies between about IX l0 and 1 X 10 0 cm." and comprising bivalent and multivalent ions anda soluble nonionic macromolecular compound having at least one sidechain of at least 4 carbon atoms.

2. In an electrophoretic method of depositing finely divided particleson an electrode in a homogeneous field, the improvement which comprisesemploying a dispersion in a mixture of isoparaffins apolar dispersingagents having an electrical conductivity of less than lO'"Q'cm. offinely divided particles capable of being deposited on an electrode byelectrophoresis and having a particle size of about 0.0l-l0um, at leastone surface-active ion-forming substance, said surface-activeion-forming substance being soluble in said dispersing agent, having adissociation capacity such that its electrical conductivity in a 10molecular solution lies between about l X l0- and l X IO OF cm. andcomprising bivalent and multivalent ions and a soluble non-ionicmacro-molecular compound having at least one side chain of at least 4carbon atoms.

1. IN AN ELECTROPHORETIC METHOD OF MANUFACTURING LUMINESCENT PICTURESCREENS THE IMPROVEMENT WHICH COMPRISES EMPLOYING A DISPERSION IN AMIXTURE OF IOSPARAFFINS APOLAR DISPERSING AGENTS HAVING AN ELECTRICALCONDUCTIVITY OF LESS THAN 10**-14 $-1 CM.-1 OF FINELY DIVIDED PARTICLESCAPABLE OF BEING DEPOSITED ON AN ELECTRODE BY ELECTROPHORESIS AND HAVINGA PARTICLE SIZE OF ABOUT 0.01-10 UM, AT LEAST ONE SURFACE-ACTIVEIONFORMING SUBSTANCE, SAID SURFACE-ACTIVE ION-FORMING SUBSTANCE BEINGSOLUBLE IN SAID DISPERSING AGENT, HAVING A DISSOCIATION CAPACITY SUCHTHAT ITS ELECTRICAL CONDUCTIVITY IN A 10**-3 MOLECULAR SOLUTION LIESBETWEEN ABOUT 1 X 10**-10 AND 1 X 10**-1 $-1 CM.-1 AND COMPRISINGBIVALENT AND MULTIVALENT IONS AND A SOLUBLE NONIONIC MACRO-MOLECULARCOMPOUND HAVING AT LEAST ONE SIDE CHAIN OF AT LEAST 4 CARBON ATOMS. 2.In an electrophoretic method of depositing finely divided particles onan electrode in a homogeneous field, the improvement which comprisesemploying a dispersion in a mixture of isoparaffins apolar dispersingagents having an electrical conductivity of less than 10 14 Omega 1cm. 1of finely divided particles capable of being deposited on an electrodeby electrophoresis and having a particle size of about 0.01-10 Mu m, atleast one surface-active ion-forming substance, said surface-activeion-forming substance being soluble in said dispersing agent, having adissociation capacity such that its electrical conductivity in a 10 3molecular solution lies between about 1 X 10 10 and 1 X 10 1 Omega 1 cm.1 and comprising bivalent and multivalent ions and a soluble non-ionicmacro-molecular compound having at least one side chain of at least 4carbon atoms.